A model of living systems must explain how an organisation maintains itself despite perturbations, exchanges with the milieu and its own transformations. This requirement does not disqualify any particular discipline. It reveals the limits of several simplifications when they become general explanations.
Causes form loops
An action modifies the state of the system, which then transforms its capacities and its future responses. Effects depend on the regime, the couplings and the scale observed. An additive analysis remains useful when interactions are weak, but it becomes insufficient once variables modify one another.
The milieu takes part in the dynamics
An organism exchanges matter, energy and information with its environment. The milieu supplies resources, imposes constraints and changes under the effect of the system. It is part of the explanation as soon as those interactions influence viability.
Stability rests on flows
Living systems keep certain variables within ranges compatible with their functioning. That stability requires energy, regulations and repairs. It can mask the progressive erosion of margins when observation is limited to current performance.
Thresholds depend on history
Wear, learning and changes of coupling modify future responses. A return to the same external parameters does not necessarily restore the earlier state. Transitions and hysteresis oblige us to integrate the trajectory followed.
Margins matter as much as performance
A local optimisation can remove redundancy, recovery time or alternative pathways. A system that performs well under ordinary conditions can become fragile when those conditions change. Viability requires tracking the capacities for repair and reconfiguration.
Measurement remains situated
A protocol chooses variables, a duration and a resolution. It can make some mechanisms visible and mask others. In living systems, observation can also modify the conduct under study. Results must remain tied to the conditions under which they were produced.
Synthesis
Viability invites us to study the loops, the milieu, the flows, the history, the margins and the conditions of measurement. These dimensions complement the analysis of components. They make it possible to specify the conditions under which an organisation maintains itself, transforms itself or disappears.